End the year with a bang: pyrotechnic lab for chemistry

End the chemistry school year with a bang!  Every year, kids always ask “are we blowing stuff up today?”  My answer is always, “we will on the last lab of the year.”  Here is how I do my pyrotechnic lab on the last lab of the year.  (I feel like a disclaimer is needed.  If you don’t feel comfortable with these demos, don’t do them.  If your school isn’t comfortable with them, don’t do them.)

Making Chinese Gun Powder

The active part of the lab is for each student to make 10 grams of Chinese gun powder.  This recipe burns more like a fountain and less like a bomb.  The kids have fun making the gunpowder and then loading it into a fun-shape fountain and then watching them burn on a board outside. Here are the methods.

7.5 g potassium nitrate

1.5 g charcoal

1.0 g sulfur

Have the students grind the potassium nitrate into a very fine powder.  Mix with the charcoal and sulfur until the mixture is even.  That’s it!

After they have made the gun powder they make a fun shape to from aluminum foil to hold it.  This year we had a cannon, a unicorn that shot fire out of it’s rear, an airplane, an elephant with a flaming trunk, and untold random shapes from the kids who “can’t do anything artistic.

The students attach a fuse that sticks directly into the gunpowder.  The powder doesn’t have to be open to the air, so long as the fuse reaches.  I buy rolls of “cannon fuse” every year from the local fireworks stand. 

Once the whole class is ready, we go outside to light them one at a time on a piece of plywood.

Pro-tip: Use plywood so the fireworks don’t melt into the asphalt in the parking lot.  Don’t ask me how I know.

Nitrogen Triiodide

After the kids light their fountains, I do a nitrogen triiodide demo.  This touch-sensitive crystal will explode with the sound of a gunshot when touched with a feather.  If you can time it right.  Nitrogen triiodide is more stable when wet and highly unstable when dry.  You have to mix up your batch a couple hours before class so it has sufficient time to dry.  This year, a went 1/3 on my chemistry classes being able to see the demo.  I do this outside and store it under a large garbage can.  But because it was windy this year, twice it went off on it’s own so the kids didn’t get to hear it.

In order to make nitrogen triiodide simply take some solid iodine crystals (I use enough to barely cover the bottom of a 250 mL beaker about 60%) and cover it with 6M ammonium hydroxide (ammonia).  Let it sit for 15-20 minutes, stirring occasionally.  I do this in a fume hood.  I use a coffee filter to filter off the liquid.  When I attach three filter papers to rings on a ring stand in a tower formation.  I smear each one with the nitrogen triiodide mixture.  Wear gloves, as it stains HORRIBLY! Then set it outside to dry.  Be aware that the paper towels and coffee filter that you use to clean up will dry in the trash can and explode later.  I tell my students and it becomes a game as they clean up their lab and throw the trash away, hoping that theirs makes the garbage can pop!

Once it has dried sufficiently outside, carefully touch the bottom filter paper with a feather attached to a meter stick, and you will get an explosion sound and a cloud of purple gas.  The gas is elemental iodine.  Do this outside as everything about this demo stains. 

Thermite

I like to make 100 grams of thermite to end the day.  I mix 75.0 grams of iron (III) oxide and 25.0 g of powdered aluminum.  Mix them thoroughly.  I buy little terra cotta flower pots at the garden store in order to do the demo.  Tape the hole in the bottom shut and place the thermite mixture in the pot.  Then I take a ring stand that fits the pot outside.  I arrange the pot over the top of a one-gallon coffee metal can loaded with dry gravel to catch the molten iron that is produced.

I don’t like being anywhere near the reaction when it goes off, so I do a secondary reaction on top to light it.  I dig a small divot in the thermite and fill it with potassium permanganate powder.  Then I drizzle some glycerin over the top.  This reaction takes 30 seconds – 1 minute to happen, giving me sufficient time to retreat.  I have the kids back a long distance.  They watch for the puff of white smoke that signals the reaction of the potassium permanganate and glycerin.  Once the smoke happens, the thermite will explode a second later with glorious sparks and liquid iron running out of the bottom of the pot onto the can of gravel.  After the reaction is over I let the kids approach so they can look at the hot iron.  It feels like you are staring at the sun because of the light and heat that comes off the molten metal.  The kids think it pretty cool!

With that, we have finally “blown something up” in chemistry.  The kids are happy.  And I have managed to tie each demo to a specific review topic.  Yes, there are dangerous elements to this lab and demos.  I take particular precautions and the kids do great.  What a way to end the year with a bang.

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